1. Field of the Invention
The present invention relates to the solvent separation of rare earth elements contained in aqueous solutions thereof, and more especially, relates to the separation, by liquid/liquid extraction, of the rare earth elements contained in aqueous solutions of the nitrates thereof.
By the expression "rare earths" according to this invention are intended the rare earth elements designated the lanthanides having atomic numbers of 57 to 71, inclusive, and yttrium, having an atomic number of 39.
In the following description of the present invention, the term "ceric rare earths" connotes the lightest elements of the rare earths, beginning with lanthanum and extending to neodymium in accordance with their atomic numbers, and the term "yttric rare earths" connotes the heaviest rare earth elements, beginning with samarium and ending with lutecium, and also comprising yttrium.
2. Description of the Prior Art
It is known to this art that the rare earths are difficult to separate in view of the fact that the differences in the properties of one rare earth relative to those of its neighbors are extremely slight.
For the separation of these very closely related elements, certain processes have been developed which have become particularly important in the separation of rare earths, i.e., the liquid/liquid extraction processes. These are based on the selective extraction of one of the rare earths from a mixed solution thereof, utilizing a solvent of extraction which is not miscible therewith.
As the extraction solvent, certain organic phosphorus compounds in solution in an organic diluent are known to this art as being useful for the separation of the rare earths from each other. In particular, it is known to thus use tri-n-octylphosphine oxide, designated TOPO.
However, TOPO is in the solid state at ambient temperature and, due to its limited solubility in the usual kerosene solvents, its use is not advisable for certain applications, in particular for the separation of rare earths in a nitrate medium and in concentrated solution.
Furthermore, certain separations, for example yttrium from other yttric rare earths, are difficult to carry out in view of their greater similarity of properties and also due to the fact that, under the conditions of the industrial decomposition of the rare earth minerals, yttrium is in the presence of relatively large amounts of yttric rare earths, which obviously is related to the composition of the rare earth mineral.